It's not that you're dialogue "sucks"- it's just oddly put together. I assume that you've played the games and are familiar with the characters you are trying to capture. You may well know that Shadow and Maria both seem to have very eloquent, flowing speech, without much slang or modern conjunction like "can't". The only real thing that I would recommend would be for you to class up the language a bit.
"Shadow," started Maria in a shushed tone, "I'll be watching you through your new life. It's a beautiful thing; I've been assigned your guardian angel. I'll help you and make sure you're okay. You don't have to remember me brother, not for me to know you love me."
That was the most in-character, touching line I saw in the chapter. It made me feel all warm and fuzzy inside. ^^
The "Bye"s at the end might be changed into "Goodbye", and that will already add a more in-character element to the fic.
However, I like the premise, and it looks like it will be very interesting. I'll keep watching! ^_~